1. Field of the Invention
The present invention relates to a variable-frequency resonator circuit, a variable-frequency filter, a shared-antenna device, and a communication device that are used, for example, in the microwave band.
2. Description of the Related Art
A variable-frequency shared-antenna device 1 having the circuit configuration shown in FIG. 8 has been known in the art. This shared-antenna device 1 has a plurality of variable-frequency resonator circuits each having a configuration in which a PIN diode is connected to a resonator via a capacitor. By controlling the voltage of these PIN diodes it is possible for a transmission circuit 25 and a reception circuit 26 to switch between two different passbands thereof.
In FIG. 8, Tx represents a transmission terminal, Rx represents a reception terminal, ANT represents an antenna, reference numerals 2 and 3 are resonators of the transmission circuit 25, reference numerals 4 to 6 are resonators of the reception circuit 26, L1 and L11 are coupling coils, C1 and C2 are coupling capacitors which determine the magnitude of the attenuation in the stop band, C5 and C6 are capacitors, L16 and L17 are resonance coils, C3 and C4 and C7 to C9 are frequency-band-varying capacitors, D2 to D6 are PIN diodes, L2 and L3 and L6 to L8 are choke coils, R1 and R2 are control-voltage supplying resistors, C22 and C23 are control-voltage supplying capacitors, L20 and L21 are coils forming a phase circuit, C15 is a capacitor forming the phase circuit, and C11 and C12 are coupling capacitors.
CONT1 is a voltage control terminal for controlling the voltage of the PIN diodes D2 and D3 in the transmission circuit 25, and CONT2 is a voltage control terminal for controlling the voltage of the PIN diodes D4 to D6 in the reception circuit 26. When a positive voltage is applied to these voltage control terminals CONT1 and CONT2, the PIN diodes D2 to D6 enter an ON state. Therefore, since the frequency-varying capacitors C3 and C4 and C7 to C9 are grounded through the PIN diodes D2 to D6, respectively, the resonance frequency is reduced and the shared-antenna device 1 operates in a LOW channel. In other words, the passbands of both the transmission circuit 25 and the reception circuit 26 shift towards the low frequency side.
Conversely, if no voltage is applied to the voltage control terminals CONT1 and CONT2, that is, if the control voltage is set to 0 V, or alternatively, if a negative DC voltage is applied to the voltage control terminals CONT1 and CONT2, the PIN diodes D2 to D6 enter an OFF state. Therefore, since the frequency-varying capacitors C3 and C4 and C7 to C9 become open-circuited, the resonance frequency increases and the shared-antenna device 1 operates in a HIGH channel. That is to say, the passbands of both the transmission circuit 25 and the reception circuit 26 move towards the high frequency side.
In the variable-frequency shared-antenna device 1 of the related art, DC control voltages for controlling the ON/OFF state of the PIN diodes D2 to D6 are applied to the PIN diodes D2 to D6 via the control-voltage supply resistors R1 and R2 and via the choke coils L2 and L3 and L6 to L8. Here, the choke coils L2 and L3 and L6 to L8 function to prevent the impedance at the voltage control terminals CONT1 and CONT2 from exerting an influence on the shared-antenna device 1. Coils having a high impedance at high frequencies may be used as the choke coils. It is necessary to use these choke coils L2 and L3 and L6 to L8 for the resonators 2 to 6, respectively. However, the size of these components is relatively large and the cost is also high. Accordingly, this has resulted in increased size and increased cost of the shared-antenna device 1.
Furthermore, the control-voltage supplying resistors R1 and R2 determine the values of the DC currents flowing in the PIN diodes D2 to D6. In order to reduce the number of components, these resistors R1 and R2 are not connected to each of the resonators 2 to 6, but rather, only one resistor is connected to each of the voltage control terminals CONT1 and CONT2. Therefore, regarding the values of the individual DC currents flowing the PIN diodes D2 to D6, the currents flowing in the PIN diodes D2 and D3, which are connected to the voltage control terminal CONT1, are identical, and the currents flowing in the PIN diodes D4 to D6, which are connected to the voltage control terminal CONT2, are identical.
Since the PIN diodes D2 to D6 are nonlinear elements, when a large electrical power is input, high-frequency signal distortion occurs, which is undesirable. In order to suppress this distortion, it is necessary to generate a large DC current flow in the PIN diodes that cause this distortion. However, in the shared-antenna device 1 of the related art, since identical DC currents flow in all of the PIN diodes D2 and D3 (or D4 to D6) that are connected to the voltage control terminal CONT1 (or CONT2), a large current also flows even in those PIN diodes that do not cause the distortion. Accordingly, a wasteful current flows, thus causing the battery of a mobile telephone terminal device to become drained quickly, which is a problem.
Moreover, in the related art, a variable-frequency resonator circuit is known in which a DC voltage for controlling a variable-capacitance diode is applied to the variable-capacitance diode via only a resistor. However, since a feature of the variable-capacitance diode is that it does not require a DC current to flow, no problems occur even though a high-impedance resistor (for example, several tens of kilo-ohms) is directly connected to the variable-capacitance diode.